A load cell is typically an electronic device, known as a force transducer, which produces an electrical signal in response to an applied force. The load cell includes a strain gauge—a device which measures deformation of an object. The electrical resistance of the object changes with deformation, and the deformation of the object depends on the force applied to the object. The load cell contains electronics for measuring the resistance of the object, thus providing an electrical signal in response to the force applied to the load cell. The load cells are calibrated to allow for ready conversion of the electrical signal to a value for the magnitude of the force applied. Load cells may comprise multiple strain gauges, allowing for the direction of the force to be resolved.
Strain gauges, such as foil gauges, can be used to measure strains up to at least 10%. Semiconductor strain gauges, such as piezoresistors, are suitable only for measurement of small strains. There are many potential applications which require measurement of larger strains. Consequently, there is a need for strain gauges for incorporation into load cells that can measure larger compressive strains—in excess of 20%.
The conventional strain gauges are large—the smallest strain gauges available today being of the order of a few millimeters square. This limits the spatial resolution for measurement of the stresses and strains. The semiconductor industry and the semiconductor tool manufacturing industry has a need to monitor the stresses and strains induced in wafers during processing and desires higher spatial resolution than is offered by prior art strain gauges. Clearly, there is a need for higher spatial resolution strain gauges.
The electrical signal produced by a typical strain gauge is of the order of millivolts prior to amplification. This can be a problem in environments with high background electromagnetic emissions. Therefore, there is a need for strain gauges that produce larger electrical signals.